This invention generally relates to actinic (deep ultraviolet, ultraviolet and visible) light sensitive positive photoresist compositions containing a mixture of an alkali-insoluble photoactive compound capable of being transformed into an alkali-soluble species upon exposure to actinic radiation, in an amount sufficient to render the mixture relatively alkali insoluble and a polymer comprising an amount of --CO--NH--CO-- groups, such as maleimide and especially maleimide - substituted styrene copolymers, sufficient to render the mixture readily alkali soluble upon exposure to actinic radiation. The invention also relates to photosensitive elements and thermally stable photochemically imaged systems based on the actinic light sensitive positive photoresist compositions.
Positive and negative photoresists are utilized in the fabrication of optical lithographic systems such as lithographic plates and semiconductor devices including integrated circuits. Generally, a photosensitive element is produced by depositing a photosensitive material on a substrate e.g., silicon or aluminum. A photochemically imaged system is produced by image-wise exposure of the photosensitive deposit or portions thereof through a patterned mask to actinic radiation to produce a latent image; the exposed deposit is treated with a suitable developer solution to form a patterned image. Whereas the exposed areas of negative photoresists are insoluble in developer, the exposed areas of positive photoresists are soluble in the developer leading to formation of a patterned image by removal of the exposed areas.
The utility of a positive photoresist composition is determined by a number of important properties. One of these properties is the inverse sensitivity which is defined herein as the actinic exposure, in terms of energy per unit area, i.e., mJ/cm.sup.2 for example, at specified wavelength or wavelengths required to completely remove film in exposed areas under specified development conditions. Other factors being the same, it is generally desirable to achieve the lowest possible inverse sensitivity since the required exposure time for a given illumination intensity (in mJ/cm.sup.2, for example) is directly proportional to inverse sensitivity and since it is usually desirable to minimize process time. Another important property of a positive resist is film loss or percent thinning which, for a given thickness, is the percent of initial film thickness which is lost in the unirradiated areas under specified exposure and development conditions. It is usually desirable that this quantity be as small as possible, other factors being the same. Typical values practiced in the electronics industry are 5-10 percent for a one micron film. A third property is contrast which relates to the ability of the resist to distinguish between different light levels. For a positive resist, contrast is defined for a set of development conditions by [log.sub.10 (E.sub.o /E.sub.s)].sup.-1 where E.sub.o is the inverse sensitivity and where E.sub.s is the exposure obtained by linear extrapolation of a plot of film thickness versus exposure from E.sub.o to full film thickness (see FIG. 1). It is usually desirable to obtain a high contrast. Typical commercial positive photoresists employed by the electronics industry which employ novolak resins give contrast values between 1.5 and 3.5 depending on development conditions. Higher contrast values have been reported for some positive photoresists based on acrylic acid containing polymers, but a complicated two-step development process is required. Although inverse sensitivity, contrast and film loss describe to first approximation the lithographic performance of a positive photoresist for a given type of exposure and given development conditions, several other properties are also important. The positive photoresist should demonstrate high quality images as demonstrated by, say, the ability to reproduce accurately and cleanly one or two micron lines and spaces in a one micron thick film. Image quality may be examined visually using optical or electron microscopy.
Since practical applications often involve subsequent processing steps at elevated temperatures, such as plasma etching, it is highly desirable that the thermal stability of the positive photoresist be sufficiently high so that the resulting relief image maintain its integrity at temperatures in the range 200.degree.-250.degree. C. A commercial product from Eastman Kodak (Micro Positive Resist 820) reportedly will withstand temperatures up to 150.degree. C. while resists from Shipley (Microposit 23) and McDermid (Ultramac) reportedly will withstand temperatures to 200.degree. C. (see Semiconductor International, April 1983, page 85-87). Hiraoka and Pacansky [J. Electrochem. Soc. 128, 2645 (1981)] have disclosed a method whereby traditional positive novolak-based resists may be flood exposed with approximately 1 J/cm.sup.2 of deep ultraviolet radiation to provide for improved thermal stability. Although good thermal stability was observed at 155.degree. C., the figures appear to show measurable flow at 210.degree. C. The procedure disclosed by Hiraoka and Pacansky requires an expensive additional processing step and results in a cross-linked relief image which is difficult to strip. Thus, while thermal stability is widely recognized as an important feature for positive photoresist compositions, there is still need for a positive photoresist composition which provides thermally stable relief images at temperatures in excess of 200.degree. C. It is also important that the positive photoresist be readily stripped following development and baking at temperatures in the range 200.degree.-250.degree. C.
It is recognized that many other characteristics of the positive photoresists may be important, depending upon the proposed application. Exemplary characteristics include good adherence to a substrate, ability to form uniform, striation-free films thereon by some process such as spin-casting, and resistance to acid etch conditions.
Typically positive photoresist compositions contain an alkali-soluble polymer such as a copolymer of formaldehyde and phenol (so called novolak) or an acrylic acid copolymer in combination with relatively large amounts, e.g., 35 weight percent of an alkali-insoluble photoactive compound such as a substituted naphthoquinone diazide which serves as an alkali-dissolution inhibitor in the positive resist composition. "Alkali solubility" as used herein will be defined in relative terms, which is to say that a highly alkali-soluble material would exhibit a dissolution rate which would result in removal of a given thickness, typically 1 micron thick film in a given amount of contact time (typically 60 seconds) at a given alkali strength. Likewise relatively alkali-insoluble material would exhibit removal of less than one half the same thickness in a similar amount of contact time at the same alkali strength. Photolysis of the positive photoresist composition is thought to cause rearrangement of the naphthoquinone diazide compounds via a ketene-containing material to indene carboxylic acid derivatives. Developing the photochemically imaged system in aqueous alkali removes the acidic phenolic resin and indene carboxylic acid, while the relatively non-polar or hydrophobic, unphotolyzed naphthoquinone diazide compounds retard or inhibit dissolution of the unexposed areas. See for example, U.S. Pat. No. 4,377,631 (M. A. Toukhy et al.) which discloses use of positive cresol-formaldehyde novolak resins with selected naphthoquinone diazide sensitizing compound. However, the novolak resins although quite useful, have some shortcomings, namely strong absorption of light in the so-called deep ultraviolet (DUV) spectral region (250-300 nm), and poor resistance of formed relief images to thermal degradation at temperatures above 120.degree.-150.degree. C.
The ultimate resolution achievable in any optical lithographic system is fundamentally limited by the wavelength of actinic radiation employed. By the term "actinic radiation" as used herein is meant electromagnetic radiation capable of causing photochemically-induced transformation. Conventional lithography utilizes actinic radiation in the ultraviolet region of the spectrum (defined herein as 300-400 nm) or in the near visible region (defined herein as 400-450 nm). Higher resolution can be achieved through use of deep ultraviolet (DUV) actinic radiation (defined herein as 250-300 nm). However, conventional, phenolic resists such as novolak resins are highly absorbing (nontransparent) in this portion of the ultraviolet region of the electromagnetic energy spectrum, and thus a flux of incident ultraviolet light sufficient for exposure is essentially prevented from reaching the lower portion of the layer of the phenolic material coated on a substrate such as a silicon wafer. For this reason, no gain in resolution upon DUV exposure, with ultraviolet radiation of wavelengths less than 300 nm, through the thickness of the phenolic material for such conventional photosensitive elements is attainable. Accordingly, such conventional positive photoresists can not form usable relief images after exposure to DUV actinic radiation.
Some DUV resists employing resins with free carboxylic acid groups are known. Thus, UK Patent Application GB No. 2,099,168 discloses a photosensitive body comprising a substrate and a photosensitive material, wherein said photosensitive material comprises an alkali-soluble polymer such as poly(methyl methacrylate-co-methacrylic acid), and an alkali-insoluble material such as a carboxylic acid ester of nitrobenzyl alcohol which upon exposure to actinic radiation becomes soluble in an alkali composition so that exposed portions of said photosensitive material dissolve in an alkali composition at a rate faster than unexposed portions of the photosensitive material and a relief image is formed.
B. E. Babb et al. (Research Disclosure, June 1975, pp. 51-53) disclose that light-sensitive triorganophosphine aromatic azide complexes in combination with binder compositions such as poly(styrene-co-maleimide) are useful in visible photographic processes and require only the steps of image-wise exposure and overall heating to 150.degree. C. for up to 5 seconds to form and stabilize the image. However, the binder merely provides physical support for the colored photochemical image species produced by transformation of the azide upon exposure to actinic radiation. There is no recognition by Babb et al. that any chemical advantage may be taken of the alkali-soluble acidic groups in the polystyrene-co-maleimide binder. In fact, the two other binders disclosed as equivalent to poly(styrene-comaleimide), namely poly(4-vinylpyridine) and cellulose acetate butyrate do not contain any acidic groups.
There is thus a need for a positive photoresist composition exhibiting low inverse sensitivity in the deep ultraviolet region of the energy spectrum, high contrast, high resolution, low film loss and the capability of forming relief images of high thermal stability.